the error: No DFU capable USB device available indicates there's some issue in communicating w/ your device over USB.
here's the info we need to help troubleshoot:
beyond that, try disconnecting all USB devices and reconnecting your Particle device. if it's connected via a hub or other extension, try connecting it directly to you computer's USB port.
from within Particle Workbench, run the Particle: Launch CLI command and then execute the following in the terminal instance that launches: particle usb list - do you see your device?
if not, try re-booting and trying again. last thing to check is your user app / firmware - in some cases a very busy loop() will prevent the device from communicating over USB. try putting your device into DFU mode manually (docs) and running the Particle: Flash application & DeviceOS (local) task.
hope it helps!